Re: [Qemu-devel] [PATCH 0/3] re-enable balloon stats

[Luiz Capitulino <lcapitulino@redhat.com>]

On Tue, 11 Dec 2012 16:28:40 -0200
Luiz Capitulino <lcapitulino@redhat.com> wrote:

> On Tue, 11 Dec 2012 11:38:08 -0600
> mdroth <mdroth@linux.vnet.ibm.com> wrote:
> 
> > On Tue, Dec 11, 2012 at 03:14:02PM +0000, Dietmar Maurer wrote:
> > > > > > I'm not sure I like this for two reasons. First, there will be cases
> > > > > > where the user doesn't want this to be enabled. Second, we'll be
> > > > > > forcing an interval on users.
> > > > >
> > > > > So when should we set the stats-polling-interval? I first thought I
> > > > > simply set that at VM start, but that triggers an error:
> > > > >
> > > > > "guest doesn't support balloon stats"
> > > > >
> > > > > because the balloon driver is not loaded.
> > > > 
> > > > Yes, it's required to have the balloon driver loaded. The stats are reported by
> > > > it.
> > > 
> > > That does not really answers my question. So you think the management
> > > framework should start the VM, and then wait until the balloon driver is
> > > loaded? That sound very clumsy to me.
> > > 
> > > I mean, I just want to set the polling interval. Why does that need the balloon drive loaded 
> > > inside the guest (polling is done by the host)?
> > 
> > I agree. Should lack of a balloon module disable the timer
> > completely, or just silently fail? Management can always reference
> > stats-last-update to check that stats are being reported properly. -1 would
> > mean driver was never loaded, longer update intervals might mean guest
> > was rebooted/suspended/etc, but in both cases it makes sense that the
> > timer still try to fetch stats, and the determination of what stats are
> > valid/invalid be left to management.
> 
> I could move the check for the stats feature bit from the function that
> enables polling to the timer callback. This would solve Dietmar's use-case,
> but it would silently fail if the feature is never negotiated (as you said
> above).

Btw, we could also add functionality to qemu-ga to list loaded modules.